Automated Material Handling Systems (AMHS) have been widely used in semiconductor fabrication facilities (“FABS”) to automatically handle and transport groups or lots of wafers between various processing machines (“tools”) used in chip manufacturing. A typical FAB generally includes one or more floors having a plurality of process bays including processing, metrology, and inspection tools and wafer staging equipment such as stockers which are interconnected by the AMHS. The AMHS is computer controlled for handling the staging of wafers for processing and flow of wafer traffic in the FAB.
Multiple wafers are typically stored and transported together in wafer carriers by the AMHS between the loadports of different wafer processing or other tools during the semiconductor fabrication process. The wafer carriers include standard mechanical interface (SMIF) pods which can hold a plurality of wafers (e.g. 200 mm or 8 inch), or front opening unified pods (FOUPs) which can hold larger 300 mm (12 inch) or 450 mm (18 inch) wafers. Typically, each wafer carrier holds on the order of approximately 25 wafers.
The AMHS in a semiconductor FAB includes numerous types of automated and manual vehicles for moving and transporting the wafer carriers throughout the FAB during the manufacturing process. This can include for example automatic guided vehicles (AGVs), personal guided vehicles (PGVs), rail guided vehicles (RGVs), overhead shuttles (OHSs), and overhead hoist transports (OHTs).
Of the foregoing AMHS wafer transport mechanisms, OHTs are commonly used to transport wafer carriers, such as FOUPs or SMIFs, from the loadport of one tool to the loadport of the next tool in the processing sequence. An OHT system includes “vehicles” that travel on an overhead monorail of the AMHS. The OHT vehicle on-board hoist is operable to raise and lower wafer carriers allowing the OHT vehicle to deposit and retrieve wafer carriers from the loadports of tools positioned along and on the floor beneath the overhead rail.
The time that it takes for the AMHS to transport wafers between semiconductor fabrication tools is one important metric since the transport time affects the FAB production efficiency, and sometimes adversely affects wafer quality in some situations. Air impurities within the FAB can contaminate wafers during transport between machines if exposed to the FAB atmosphere for too great a period of time, particularly since some wafers can be more sensitive to contaminants depending on the type of fabrication process just completed in the tool. Accumulation time, or “Q time” for short, is a measure of the time that it takes for a wafer to travel from the loadport of a first tool to the loadport of the second tool in the fabrication sequence. Accordingly, it is a general goal to keep Q time as short as possible for both potential wafer contamination and fabrication efficiency reasons. The AMHS, however, is susceptible to OHT vehicle bottle necks and “traffic jams” which delays wafer transport between tools and reduces Q time.
An improved system and method for transporting wafers between semiconductor fabrication process tools is desired.
All drawings are schematic and are not drawn to scale.